Relatively large areas, such as multiple building campuses, airports and the like, are conventionally secured against undesired entry by way of fencing or other barriers around the perimeter of the secured area. Particularly where portions of the area are not subject to constant human surveillance, either directly by watchpersons or indirectly by camera, remote detection of intrusion or other breach of the perimeter allows deployment of the necessary security personnel as needed. In this way, effective asset protection can be efficiently maintained with relatively few security personnel.
An example of a conventional remote intrusion detection system is the VTW-300 electronic taut wire fence manufactured and sold by Vindicator Corporation. In this system, multiple taut wires make up a perimeter fence. Sensors are connected to the taut wires to sense their deflection, such as may result from an intruder climbing the fence, and to generate an electrical signal to a data processing system for communication of the appropriate alarm or alert condition to security personnel. The security personnel can then initiate the appropriate response to the detected condition.
In this prior system, a strain gage sensor consisting of a single resistive element is used to convert the mechanical motion of the taut wire deflection into an electrical signal. Referring now to FIGS. 1a through 1c, an example of a conventional sensor, namely the model STW-30 taut wire deflection sensor manufactured and sold by Vindicator Corporation, will now be described. This sensor includes a rubber cylindrical housing 2 having slotted bolt 4 mounted therewithin and extending therefrom; the taut wire is secured within slot 5 by a nut (not shown). Housing 2 includes a rectangular cavity 7 into which strain gage assembly 6 is disposed in a cantilever fashion. Sealant 11 secures strain gage assembly 6 within cavity 7; adhesive seal 17 and sealant 11 together protect assembly 6 from moisture and other environmental effects. Housing 2 is mounted to plate 9, which can be mounted by way of bolts or screws to a fence post, wall, or to a sensor post which in turn is mounted to a fence post or wall.
In this prior sensor, referring in particular to the cross-sectional view of FIG. 1a, strain gage assembly 6 is implemented in the conventional manner to measure the strain of a metal member. In the sensor of FIG. 1a, this metal member is metal substrate 15, formed of an alloy such as beryllium-copper, and which is relatively thin so that it can flex in a direction normal to its plane. Resistive element 14 is formed of conventional strain gage material such as an alloy of copper-nickel, nickel-chromium, platinum-tungsten, or platinum-iridium, formed in a serpentine pattern so that its length is significantly greater than its width, and arranged so that flexing in a direction normal to its plane will modulate its DC resistance. Resistive element 14 is applied onto insulating film 13, for example "KAPTON" polymer tape, which in turn adheres to metal substrate 15; insulating film 13 thus electrically insulates resistive element 14 from metal substrate 15. Wires 16 are connected to resistive element 14 and extend from assembly 6 out of sealant 11 and seal 17. As illustrated in FIGS. 1a and 1c, groove 10 encircles housing 2 at a position matching the position of resistive element 14. As such, groove 10 focuses any bending motion of housing 2 resulting from tension on a taut wire connected to slotted bolt 4 to the location of resistive element 14 in strain gage assembly 6, increasing the sensitivity of the sensor.
As indicated in FIG. 1a, resistive element 14 provides a single resistance value. As is well known, flexing of or other strain upon resistive element 14 in a direction normal to its plane will change its resistance value. Electrical circuitry (not shown) can thus determine the strain applied to resistive element 14 by measuring its resistance, for example by measuring a voltage drop thereacross for a known current. In the conventional VTW-300 system noted hereinabove, the resistance of element 14 is measured by conventional analog circuitry within a data processing system coupled to wires 16.
It has been found, however, that the conventional strain gage assembly 6, including single resistive element 14, is subject to temperature instability, as the resistance of resistive element 14 changes with temperature. Such instability can cause false alarms to be issued due to temperature changes; worse yet, tension on the taut wire may not result in an alarm condition if the resistance value has changed, due to temperature, so as to be more tolerant of deflection.
By way of further background, another conventional sensor for detecting taut wire deflection has the taut wire connected to a switch mounted within a deformable plastic, in particular a plastic which deforms slowly responsive to a force so that the switch self-centers. The self-centering action thus compensates for slow drift in the switch position due to temperature and other environmental changes. This configuration, however, generates only a digital (open/closed) output, and provides no way of adjusting its sensitivity or response time.
By way of further background, full bridge foil strain gages are well known in the prior art. Examples of conventional full bridge foil strain gages include the "OMEGA" VY 11 90.degree. full bridge cluster, and the 100 QC-350 foil strain gage available from Micro Engineering 2, a division of JP Tech. These conventional strain gages are readily available on insulating films such as "KAPTON" polymer tape, for application to metal members as described hereinabove.
It is therefore an object of the present invention to provide an intrusion detection sensor, such as one adapted for taut wire deflection sensing, having improved temperature stability.
It is a further object of the present invention to provide such a sensor which provides improved signal/noise ratio communication to the data processing system, and improved immunity to noise.
It is a further object of the present invention to provide a strain gage assembly for such a sensor having improved reliability and reduced manufacturing complexity.
It is a further object of the present invention to provide such an assembly which provides flexibility in its sensitivity and response characteristics.
It is a further object of the present invention to provide such an assembly which is capable of determining the direction of deflection so that, when used in an intrusion detection system, the location of the intruder may be determined.
Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings.